Method and apparatus for measuring the filling effectiveness of a cable during filling

ABSTRACT

The invention relates to the monitoring of the filling effectiveness during the filling operation of a waterproof or filled telecommunications cable on a manufacturing line, and includes measuring the capacitance change per unit length of an outer pair of insulated conductors in the cable, measuring the capacitance change per unit length of an inner pair of insulated conductors in the cable, determining any deviations in the measured capacitance changes and utilizing such deviations by feedback control to eliminate further deviation, as well as determining the point of deviation along the cable length.

RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 642,852, filedDec. 22, 1975 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to method and apparatus for measuring the fillingeffectiveness of the filling operation in the manufacture of waterproofcables, wherein the capacitance between an outer pair of conductors ismeasured and the capacitance between an inner pair of conductors ismeasured, and the capacitances are compared.

2. Description of the Prior Art

Telecommunications cables, especially those that are to be buried in theground, are desirably moisture proofed to prevent transmissiondifficulties resulting from the seepage of moisture into the cable. Ingeneral, such moisture proofing is accomplished during manufacture ofthe cable by filling the internal volume of the cable with a suitablefilling compound, such as, for example, petrolatum or a mixture ofpetrolatum and polyethylene. For the desired results to be achieved, thefilling material should preferably occupy substantially all of thevolume of the cable that is unoccupied by the conductors and othercomponents therein, including the interstices between twisted pairs ofconductors. Various methods and apparatus for filling cables are shown,for example, in U.S. Pat. Nos. 3,832,215, 3,854,444, and 3,850,139 ofFranke et al., 3,789,099 and 3,876,487 of Garrett et al., 3,733,225 ofMoody, and in copending U.S. Pat. application Ser. No. 457,877 ofFreeman et al., filed Apr. 4, 1974, and assigned to the presentassignee, now abandoned.

The normal filling procedure involves the introduction of the fillingcompound after the core has been formed and before the final binder andsheath are placed on the core. At this stage of manufacture, the core isrelatively compact and it is difficult to introduce the fillingcompound, yet the prevention of the ingress of moisture in subsequentuse requires that there be a high percentage of fill in the totalfillable volume, preferably evenly distributed throughout the cablecross-section.

Numerous arrangements for ascertaining the amount or percentage of fillmaterial in a cable, which is an indication of filling operationeffectiveness, have been devised. One such arrangement comprises cuttingoff an end portion of a finished cable and subjecting one end thereof towater under a known pressure. If more than a predetermined amount ofwater flows out of the other end, the cable is unacceptable. Anotherarrangement comprises weighing a short length of filled cable. Since theunfilled weight is known, and the weight of the proper amount of fillmaterial for such a length can be determined, the weight of the filledlength of cable should at least equal the sum of the two to beacceptable.

Still another method for determining the acceptability of the filledcable comprises measuring the capacitance of a number of pairs of outerconductors in a finished cable, then measuring the capacitance of anumber of pairs of inner conductors, and comparing the two measurements.The difference between the two measurements, divided by the outermeasurement provides a measure of the filling effectiveness which canthen be compared to empirically predetermined values to ascertainwhether or not the cable is acceptable.

In the prior art methods of determining filling effectiveness, examplesof which are given in the foregoing, the operations are performed on afinished cable, hence if the filling effectiveness is found to beinadequate, a whole cable run must be scrapped or attempts made torefill the cable. In those processes where the measurements or tests aremade on a short length of cable, there is no way of determining whetherthe remainder of the cable is the same as the tested sample, hence acalculated risk is taken in depending on the test results. In thosearrangements where the entire cable length is tested, as in thecapacitance measuring method, an indication of non-acceptability mayresult from only a very short faulty length of cable, which could be cutout if its location along the cable length were known.

This latter problem is common to virtually all of the prior artarrangements, namely, there is no way of ascertaining where, along thecable length, the amount of fill has fallen below an acceptable minimum.An additional drawback of prior art testing methods is that they areperformed on finished cables, and unacceptable cables must be scrappedor refilled, which entails both extra time and money.

SUMMARY OF THE INVENTION

The foregoing problems are overcome by the method and apparatus of thepresent invention, wherein the method includes the steps of continuouslymonitoring the change in capacitance of an outer pair of conductors asthe cable passes through the filling stage; continuously monitoring thechange in capacitance of an inner pair of conductors as the cable passesthrough the filling stage; comparing the monitored changes incapacitance with each other to ascertain the filling effectiveness ofthe filling operation; and providing an indication of the location alongthe cable length of points or regions where deviations in fillingeffectiveness occur.

By means of the foregoing steps, the location of unacceptable regions offill are pinpointed, which regions result from a drop or decrease infilling effectiveness of the filling operation. Filling effectiveness,in this context, is simply the ratio of volume actually filled to totalfillable volume, or, in terms of cross-section, the ratio of actualdistribution of fill in the cross-sectional area to total fillablecross-section.

Because of the continuous monitoring of the capacitance change asembodied in the foregoing steps, it is possible, utilizing the presentinvention, to control the filling operation to remedy defects in theoperation and virtually assure maintenance of acceptable fillingeffectiveness during the manufacturing run. Thus, the method of theinvention may include the additional steps of generating control signalsin response to deviations in filling effectiveness to vary a parameterof the filling operation to correct such deviations. These parametersinclude the temperature and pressure of the filling compound, and theline speed of the moving cable as it passes through the filling stage.

DESCRIPTION OF THE DRAWINGS

The invention, and its mode of operation, will be more fully understoodby reference to the following detailed description and to the drawings,in which:

FIG. 1 illustrates a portion of the cable core, filling equipment, afootage counter, and monitoring equipment is schematic form;

FIG. 2 illustrates the monitoring equipment in diagrammatic form and theinterconnections with the cable core;

FIG. 3 illustrates capacitance measuring circuitry;

FIG. 4 illustrates encoder circuitry, a transmitter and receiver anddecoding circuitry utilized in practicing the invention;

FIG. 5 illustrates waveforms which are present at various points in themeasuring circuitry;

FIG. 6 illustrates a computer flow chart practiced in the invention; and

FIG. 7 illustrates a graph of filling effectiveness for a particulartype of cable.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, there is shown a portion of a cable core 1,the major portion of which is shown on a pay-off reel 2, which isrotatable on a shaft 14. The core 1 is shown passing through a fillingchamber, indicated generally by the numeral 3, and which is of the typedescribed in copending U.S. application Ser. No. 457,877, referred topreviously. The core 1 advances from the filling chamber 3, over afootage counter 4, and is then taken up on a take-up reel, not shown,which is driven by suitable drive means 10. Between the filling chamber3 and the footage counter 4 various other manufacturing operations maytake place, such as a final binder spirally applied to the core, analuminum sheath applied over the binder, and an insulating jacketextruded over the sheath, none of which are shown, but all of which arewell known operations in the manufacture of telecommunications cables.

In FIGS. 1 and 2, it may be seen that the cable core 1 includes, in thisembodiment, a plurality of twisted pairs of insulated conductors 6. Anouter twisted pair of conductors 7 are connected in any suitable manner,as, for example, by alligator clips to a pair of leads 8 and by them tothe input of a capacitance measuring circuit, encoder, and transmitter,which is indicated generally as block 9, and which is disclosed indetail in FIGS. 3 and 4. The circuitry of block 9 is connected by outputleads 11 to an input coupling coil 12, which is rotatable with the reel2.

The input coupling coil 12 is associated with an output coupling coil13, which is stationary on the shaft 14. The coil 13 is connected byleads 16 to a receiver and decoder which is indicated generally as theblock 17, and which is disclosed in detail in FIG. 4.

In a similar manner, an inner twisted pair of insulated conductors 18(see FIG. 2) are connected in any suitable manner, as, for example, byalligator clips to a pair of leads 19 and by them to the input of acapacitance measuring circuit, encoder, and transmitter, which isindicated generally as the block 21, and which is similar to thecircuitry disclosed in the block 9 in FIGS. 3 and 4. The circuitry ofblock 21 is connected by output leads 22 to the rotatable coupling coil12, described previously. The signals from the leads 22 are coupledthrough the coil 12 to the stationary coupling coil 13, and throughleads 23 to a receiver which is indicated generally as the block 24, andwhich is similar to that disclosed in detail in the block 17 in FIG. 4.

It may be further seen that the outputs of the receivers 17 and 24 arefed to a computer or processor 26, which could be a general purposedigital computer, for a purpose to be described in detail subsequently.

The capacitance monitoring circuitry contained in the blocks 9 and 17and depicted in FIGS. 3 and 4 represents an arrangement for achieving ahigh degree of accuracy in the monitoring process. It is to beunderstood, however, that other circuit arrangements for monitoringcapacitance changes might also be used, depending upon the degree ofaccuracy and speed of response desired. While the following descriptionis directed to the circuits of blocks 9 and 17 for monitoring thecapacitance change between the conductors 7, substantially identicalcircuitry is represented by blocks 21 and 24 for monitoring thecapacitance change between the conductors 18. Because coils 12 and 13are common to both monitoring branches, the circuitry of blocks 21 and24 operates at different frequencies from that of blocks 9 and 17.

As the filling operation progresses, the mutual capacitance of the pairof conductors 7 will increase because the air between conductors 7,which has a dielectric constant of 1.0, is replaced by the fillingcompound, which has a dielectric constant materially different from thatof air, such as, for example, 2.2. In addition, as the filled length ofthe cable core increases, the capacitance also increases as a functionof length. The monitoring equipment is operated on the principle thatunder routine operating conditions, the outer pair of conductors 7 willbe approximately 100% surrounded with the filling compound because theyare at the outside of the cable core 1 as it passes through the fillingchamber 3.

In the circuit of FIG. 3, a voltage reference source 31 generates anoutput, preferably direct current, such as a positive 5 volts, which isapplied to an inverting amplifier 34 via lead 32, and to one contact 42of a single pole-double throw switch 39. The output of amplifier 34 isapplied by lead 37 to the other contact 38 of switch 39. Switch 39 maytake any of a number of suitable forms, such as, for example, a solidstate device. The voltages applied to contacts 42 and 38 are depicted inFIG. 5 as 33 and 36, respectively.

Contactor 43 of switch 39 applies either the positive (33) or negative(36) reference voltage to a buffer amplifier 46 via lead 44. As will beapparent hereinafter, a waveform such as 45 in FIG. 5 can be made toappear on lead 44 and the output lead 47 of amplifier 46 by a periodicactuation of switch 39. The output of amplifier 46 is applied to thenegative input of a difference amplifier (constant current generator)48, the output of which is applied through a charging resistor 49 to theconductors 7, the capacitance between which is then charged (anddischarged). Amplifier 48 adds a small amount of gain to the referencevoltage input so that the capacitance can be charged to voltages eitherhigher or lower than the positive and negative reference voltages,respectively.

The charging (and discharging) of the capacitances is monitored by abuffer amplifier 51, which serves to isolate the capacitance chargingcircuitry from the loading effects of other parts of the circuit. Theoutput of amplifier 51, represented by curve 55 in FIG. 5, is appliedvia lead 66 to the plus or positive input of amplifier 48 so that, asthe amplifier 48 monitors the difference between its two voltage inputsit provides through resistor 49 a constant current charging ordischarging of the capacitance of the conductors 7.

The output of amplifier 51 is also directed through lead 52 to a voltagedivider, made up of resistors 53 and 54, the output of which,represented by curve 59 in FIG. 5, is applied to one input of each of apair of comparators 57 and 58. Comparators 57 and 58 also have appliedto their inputs the positive and negative reference voltagesrespectively, over leads 41 and 37, as shown. In addition, the output ofamplifier 51, as represented by curve 55 of FIG. 5, is applied to oneinput of each of a pair of comparators 63 and 64, whose other inputshave applied thereto the positive and negative reference voltages overleads 41 and 37, respectively.

The outputs of comparators 57 and 58 are applied to a flip-flop circuit61, whose output is used to control switch 39. Where the input waveform59 (FIG. 5) to comparator 57 equals or is greater than the positivereference voltage on lead 41, the comparator 57 produces an output toset the flip-flop 61 and in turn activate switch 39 so that contactor 43engages contact 38, and the negative reference voltage is applied toamplifier 46. Conversely, if the input waveform 59 (FIG. 5) equals or ismore negative than the input on lead 37 to comparator 58, comparator 58generates a signal to reverse the flip-flop 61 and hence switch 39,thereby applying the positive reference voltage to amplifier 46. Throughthe action just described, the waveform 45 of FIG. 5 is generated andapplied to difference amplifier 48.

It can be seen that the circuitry thus far described monitors thecharging of the capacitance of conductors 7 until the charge reaches aspecified reference level, then causes the capacitance to discharge andrecharge in the opposite direction to a specified reference level. Inorder to obtain a proper evaluation of the change in capacitance, it isdesirable to monitor the time span of the charging and dischargingcycles. This is accomplished in the arrangement of FIGS. 3 and 4 bycomparators 63 and 64 and associated circuitry.

As described previously, the output of buffer amplifier 51 follows thecharging and discharging of the capacitance of the conductor pair 7, theresulting waveform being represented by curve 55 of FIG. 5, and appliesits output to the comparators 63 and 64. The outputs of comparators 63and 64 are applied to the two inputs of a NOR gate 67, as shown. Absentany signal to either of its inputs, NOR gate 67 supplies a true or onsignal in a known manner, but when a signal appears at either input, thegate shuts, or otherwise indicates an off condition. When the signalapplied to comparator 63 from amplifier 51 is less than that on lead 41,comparator 63 produces no output. In like manner, when the signal fromamplifier 51 to comparator 64 is greater than that on lead 37,comparator 64 produces no output. Under these conditions, NOR gate 67gives an on indication. However, when the input to comparator 63 fromamplifier 51 equals or exceeds the signal on lead 41, comparator 63produces an output which switches NOR gate 67 off. By the same token,when the signal from amplifier 51 to comparator 64 equals or is lessthan that on lead 37, comparator 64 produces an output which turns gate67 off. Thus when waveform 55 of FIG. 5 is applied to comparators 63 and64, the resulting output of NOR gate 67 is represented by waveform 68 ofFIG. 5, with the length or duration of the charging cycle being given bythe on period T. It can be appreciated that as the cable is filled, theperiod T will increase, due to the increased capacitance and hence theincreased charging and discharging times, which decreases the slopes ofwaveforms 55 and 59.

In FIG. 4 it can be seen that the output of NOR gate 67 is applied toone input of an AND gate 69, whose other input has clock signals appliedthereto from a crystal oscillator clock 70. The output of gate 69 isapplied to a binary counter 71. It can be seen that during each period Tof waveform 68 of FIG. 5, i.e., when NOR gate 67 is giving a true or onindication, a series of digital pulses at the clock frequency areapplied to counter 71, which counts the pulses and outputs to a shiftregister 72 binary numbers indicative of the length of the period T. Atiming and control circuit 73 which receives signals over lead 65 fromflip-flop 61 resets counter 71 at each change of condition of flip-flop61, and at the same time empties shift register 72 in a serial datastream to a variable modulus divider 74. Thus the counting cycle ofcounter 71 is made to coincide with the charging and discharging cyclesof the capacitance being monitored. Further, the actual count itselfindicates the length of the charging or discharging cycle, and changes(increases) as the filling operation progresses.

Variable modulus divider 74 receives an input from clock 70 as well asfrom shift register 72, and produces a pair of output frequencies, suchas 6.25 KHZ and 5.68 KHZ, one of which represents binary 1's of thesignal from the shift register and the other of which represents binary0's of the same signal. The output of divider 74 is passed through a lowpass filter 76 to the rotatable, coupling coil 12 as signals indicativeof the charging capacitance of conductors 7.

At this stage of the operation of the monitoring system illustrated inFIG. 1, there have been created audio frequency signals which indicatethe changing capacitance of conductors 7 as the filling operationprogresses. In a like manner, similar signals will have been generatedby the circuitry of transmitter 21 to indicate the changing capacitanceof conductors 18. It is possible to operate with these signals toachieve the desired comparisons and hence a measure of the fillingeffectiveness in a number of ways. The remaining circuitry of FIG. 4illustrates one arrangement for achieving the desired results.

The audio frequency signals in coil 12 are picked up by coil 13 andapplied via leads 16 to a band pass filter 77. Filter 77 functions topass those frequencies indicative of the capacitance and capacitancechanges of conductors 7. A similar filter in receiver 24 passes onlythose frequencies indicative of the capacitance and capacitance changesof conductors 18.

The filtered signal is applied to a converter 78 which generates avoltage output having a magnitude determined by which frequency (6.25KHZ or 5.68 KHZ) is applied to its input. The output of the converter isapplied to a voltage comparator 79 which generates a binary numberindicative of which voltage was received at its input, and its binaryoutput is applied to a shift register 81. The comparator 79 and theshift register 81 continuously receive the asynchronous serial datatransmission from the transmitter.

The output of comparator 79 is also applied to a synchronization logiccircuit 83 which recognizes when a complete signal word is present inshift register 81 and signals a data latch circuit 82, connected to theoutput of register 81, to store the word. The latch circuit thengenerates a read command signal which is applied through lead 85 tocomputer 26, and the computer reads and stores the signal input from thelatch circuit applied over leads 86. The binary signals received bycomputer 26 over leads 86 are indicative of the capacitance changebetween conductors 7 as the filling operation progresses. Footagecounter 4 (FIG. 1) also applies signals over leads 88 to computer 26. Atthe same time, signals representing the capacitance change betweenconductors 18 are applied to the computer 26 over leads 87. The signalfrom counter 4 is preferably a pulse per distance indication, such as,for example, one pulse per foot of cable 1 passing over it. Counter 4may be any one of a number of types well known in the art, such as, forexample, the type shown in U.S. Pat. No. 2,783,540 of Berry, or asuitable one of the several types mentioned in column 1, lines 25through 30 of that patent. Alternatively, counter 4 may be of a typesimilar to that shown in the aforementioned U.S. Pat. No. 3,733,225 ofMoody.

The operation of the computation steps performed by computer 26 can bestbe understood with reference to FIG. 6, a computer flow chart. As waspointed out in the foregoing, the signals applied to coil 12, along withthe footage signals, contain the necessary data for computing thefilling effectiveness of the filling operation. The circuitry ofreceiver 17 (FIG. 4) is designed to prepare this information for use bycomputer 26, but it is to be understood that the following operationscould be performed by means other than a computer, if desired.

In the flow chart of FIG. 6, box 91 represents the data inputs tocomputer 26. The computer 26 then determines the increase of capacitanceper unit lengths of cable 1 processed, or the slope, for the outsidetwisted pair of conductors 7 and for the inside twisted pair ofconductors 18, by dividing the change of capacitance by the change ofprocessed footage of filled cable, as shown in box 92 of FIG. 6.

The computer 26 then compares the derived slope of the capacitance ofthe outside pair of twisted conductors 7 to a predetermined slope valueand calculates the percent difference in capacitance between the two, ifany. The predetermined slope value is figured on the basis of a filledcable having an average mutual capacitance of 83 nanofarads per mile oflength. This is shown as box 93 of FIG. 6.

The computer 26 also performs the same computations with respect to theinside pair of twisted conductors 18, to determine the percentdifference in capacitance, but comparing with the outside paircapacitance slope value, as shown as box 94 of FIG. 6.

The computer 26 then is used to determine or calculate the fillingeffectiveness for the outside pair of twisted conductors 7, as shown asbox 96 of FIG. 6.

The filling effectiveness is determined from the mutual capacitancedifference, and is a function of two significant variables. The firstvariable to be considered is the geometric spacing of the two insulatedconductors 7 with respect to each other and with respect to the otherinsulated conductors 6 in the cable 1. The second variable is thedielectric constant of the insulating material surrounding theconductors and entering the interstices therebetween.

However, from a practical standpoint, it may be assumed that thevariable of the geometric spacing will remain relatively constantthroughout the cable filling process, and so may be assumed constant inthe computations. This, then, leaves the dielectric constant of theinsulating material surrounding the conductors to be taken intoconsideration, but must be computed in terms of mutual capacitance.

The filling effectiveness, as mentioned earlier, is defined as anindication of the fillable cross-sectional area which has been filledwith waterproofing compound as compared to the total cross-sectionalarea that could be filled to result in 100% fill.

Further, the portion of fillable area filled with the compound relativeto the total fillable area is a function of the dielectric constant ofthe total fillable area.

Thus, the filling effectiveness may be determined by using the followingequation: ##EQU1## Where: E_(F) = Dielectric Constant of Fillable Areaof Cable

E_(pj) = dielectric Constant of Pure Filling Compound.

Then by measuring the change of this dielectric constant the amount offilling compound which has been added may be determined.

However, this change cannot be measured directly, but an equation mustbe used which relates this change to total change in mutual capacitance,which can be measured. Such an equation is: ##EQU2## Where:E_(F).sbsb.Max = Maximum value of E for 100% filled

E_(f).sbsb.min = Air ≈ 1.00

E = overall dielectric constant

E_(i) = dielectric constant of insulation on conductors ##EQU3##

If the value, or equation, for E_(F) is substituted in the equation setforth previously, it will be possible to solve for the fillingeffectiveness.

However, the first set forth equation may be further simplified, using aspecific type of cable.

As an example, in testing one type of cable in manufacture, such aspolypropylene insulated conductors and the cable having a mutualcapacitance of 83 nanofarads per mile, the equation will be: ##EQU4##Where: ΔE = Difference in capacitance between the outside pair and apredetermined value or difference in inside and outside capacitances(Boxes 93 and 94, FIG. 6).

With the cable of the present example, the percent ΔE is limited to therange 0-16.

All of the above computations are indicated as being performed in boxes96 and 103, FIG. 6.

FIG. 7 is a curve illustrating the above equation for fillingeffectiveness as a function of the percent difference in mutualcapacitance, for the type of cable mentioned above.

As mentioned previously, under normal operating conditions it is assumedthat the outer twisted pair of conductors 7 receive 100% fill. If thisis in fact occurring when the slope is calculated in box 93, FIG. 6, thefilling effectiveness will be calculated with the resultant calculationbeing equal to a maximum 100% in box 96. However, if this does not occurthe filling effectiveness will be calculated, box 96, FIG. 6. If theresult is less than the predetermined value the computer 26 will supplysignals over a pair of leads 98, FIG. 1, to control a valve 99 in thefilling chamber 3 to increase the pressure of the filling compound toattempt to reach the predetermined fill condition on the outer twistedpair of conductors 7. Such is shown as box 101 in FIG. 6. The valve 99may be a digital flow valve such as Model 6-607D of Digital Dynamics,Inc. of Sunnyvale, Cal.

Further, the computer 26 also may signal over a pair of leads 102, FIG.1, to increase the temperature to the filling chamber 3 to cause thefilling compound to be less viscous. Still further it is possible forthe computer to generate signals for controlling drive means 10 throughleads 110 to alter the line speed of the advancing cable 1.

Obviously, the computer 26 may generate signals to control either one,or combination, or all of the above mentioned variables, as it iscontinuously monitoring the relative filling effectiveness.

The computer 26 also calculates the filling effectiveness on the insidetwisted pair of conductors 18, using the equations set forth above, asshown in box 103, FIG. 6. The calculated value may not be the same ascalculated for the outside twisted pair of conductors 7 (box 96, FIG.6), as the slopes may be different (see boxes 93 and 94, FIG. 6).

In the event that the calculated value is determined to be less than apredetermined requirement as determined in box 104, FIG. 6, the computer26 will generate signals to control the variables, box 106, FIG. 6.These signals will be similar to those generated in box 101, FIG. 6, tosimilarly control the pressure or temperature of the filling compound orthe line speed of the cable 1.

During the operation of the equipment the computer 26 will send signalsover a pair of leads 107 to cause the results of the continuousmonitoring of the filling effectiveness to be recorded on a recordingdevice 108. The recording may be a series of actual value readings on aprint-out with corresponding footage values of filled cable core 1, ormay be a plotting of data and footage, such as that calculated in theboxes 96 and 103, FIG. 6.

In the alternative, it is possible to obtain the filling effectivenessof the cable, but not as precisely as described above, by measuring thecapacitance change of a single twisted pair of conductors, preferablynear the center of the cable, such as the twisted pair 18. The signalsindicative of such capacitance would be handled in a manner similar tothat described above and placed in the computer 26. The computer 26would also have a standard capacitance change stored therein for theparticular type of cable being filled, and would process the measuredchange as by using boxes 93, 96, 97, 101, and 108, FIG. 6.

It is to be understood that the above-described arrangements are simplyillustrative of the principles of the invention. Other arrangements maybe devised by those skilled in the art which will embody the principlesof the invention and fall within the spirit and scope thereof.

What is claimed is:
 1. A method of monitoring the amount of fill beingplaced in a cable as the cable is advanced through a filling chamber,which comprises the steps of continuously monitoring the changingcapacitance of a twisted pair of conductors in the cable, and comparingthe monitored changes in capacitance to a predetermined standard valueof capacitance change.
 2. A method of measuring the fillingeffectiveness of the filling operation in the manufacture of waterproofcables having a plurality of conductors therein, which comprises thesteps of:continuously monitoring the change in capacitance of an outerpair of conductors as the cable passes through a filling chamber,continuously monitoring the change in capacitance of an inner pair ofconductors as the cable passes through the filling chamber, andcomparing the monitored changes in capacitance of the outside pair andthe inside pair with each other to ascertain the filling effectivenessof the filling operation.
 3. The method according to claim 1, andfurther including the step of providing an indication of the locationalong the cable length of regions where deviations in fillingeffectiveness occur.
 4. The method according to claim 1, and furtherincluding the step of comparing the monitored capacitance change of theouter pair of conductors to a predetermined reference value. 5.Apparatus for measuring the filling effectiveness of the fillingoperation in the manufacture of waterproof cables having a plurality ofconductors comprising:first means for continuously measuring the changein capacitance of an outer pair of conductors in the cable as it passesthrough a filling chamber, second means for continuously measuring thechange in capacitance of an inner pair of conductors in the cable as itpasses through said filling chamber, and means associated with saidfirst and second means for generating signals indicative of the changesin capacitance of the conductors as the changes occur.
 6. Apparatusaccording to claim 5 and further comprising:means for determining thefilling effectiveness including means for comparing the change incapacitance of the outer pair of conductors to the change in capacitanceof the inner pair of conductors.
 7. Apparatus for measuring the fillingeffectiveness of the filling operation in the manufacture of waterproofcables having a plurality of conductors comprising:means including avoltage source for continuously charging and discharging the capacitanceof a pair of the conductors between upper and lower voltage limits;comparator means responsive to the charge on the capacitance reaching acharging limit for generating a signal to reverse the voltage applied tothe capacitance; second comparator means for providing an indicationwhen the charge on the capacitance exceeds the upper and lower voltagelimits; means for generating a pulse train in the absence of such anindication from said second comparator means; and means for counting thepulses generated between such indications, the number of pulsesindicating the charging and discharging times of the capacitance. 8.Apparatus according to claim 7, wherein the pair of conductors is afirst pair of conductors, and further including:means for continuouslycharging and discharging the capacitance of a second pair of conductorsbetween upper and lower voltage limits; third comparator meansresponsive to the charge on the capacitance of the second pair ofconductors reaching a charging limit for generating a signal to reversethe voltage applied to the capacitance; fourth comparator means forproviding an indication when the charge on the capacitance of the secondpair of conductors exceeds the upper and lower voltage limits; means forgenerating a second pulse train in the absence of such an indicationfrom said fourth comparator means; and means for counting the pulsesgenerated between such indications, the number of pulses indicating thecharging and discharging times of the capacitance of the second pair ofconductors.
 9. Apparatus according to claim 8 and furtherincluding:means for converting the pulses into signals indicative of thechanging capacitance of the first pair of conductors and of the secondpair of conductors, and means for comparing the signals representativeof the changing capacitance of the first pair of conductors to thesignals representative of the changing capacitance of the second pair ofconductors for determining the filling effectiveness of the fillingoperation.
 10. Apparatus according to claim 9 wherein saidlast-mentioned means includes means providing an indication ofdeviations in filling effectiveness.
 11. Apparatus according to claim 10and further including means for monitoring the cable travel through afilling chamber to provide an indication of where along the cable lengthdeviations in filling effectiveness occur.